This invention provides a means of securing two doors to each other in a way that provides greatly enhanced security for people in a dwelling in a simple, convenient and inexpensive manner. The invention is generally applicable to situations in which there is an outer door in the same frame with an inner door each with their separate hardware and locks.
In the ordinary situation the outer door may be a second door or it may be a screen or storm door. Generally the outer door is intended to protect the inner door from the weather and to provide an insulating and sound deadening effect. The outer door is generally equipped with its own doorknobs and lock, though in practice they are frequently of inferior and inadequate locking construction and are rarely used. Sometimes they have a latch type device rather than a standard key operated lock which serves to keep them from blowing open.
Generally the inside door is the main structural member for the dwelling, having its own doorknobs and lock and the inside door normally has a more expensive key operated lock for security. The inside doors are generally of better and thicker construction with better locks yet many of the locks are easily opened with a plastic card or a skeleton key. Even so, once the outer door is opened, the inner door is subject to being kicked in or broken into by a large man running his shoulder into it.
Many people have turned to installing additional locks on their inner door which are specially designed to resist opening with skeleton keys or plastic cards. Some have installed chains or bars of various types for greater security. Very heavy chains and bars are required to substantially enhance the impact resistance of a standard door so as to provide enough protection and the special locks tend to be expensive and require professional installation. Many of these devices are cumbersome and difficult to use because they tend to jam and stick. This is particularly true of the sliding bolt type security devices. In addition many of them are difficult to find and open under poor lighting conditions that might be typical of an emergency situation.